Plaque pH and associated parameters in relation to caries

Intensified plaque acidogenicity in caries-prone subjects was reported many years ago, but emerging evidence has suggested that the relationship may n ot be as strong as once thought. We have now determined a range of acidogen icity variables in subjects having both caries prevalence and incidence dat a, and have included plaque mineral data in the analysis. pH measurements w ere made in 20 randomly selected subjects from a high-caries group (mean DM FS = 8.95) and 20 from a caries-free group of Beijing children aged 12 year s participating in a caries prediction study. Subgroups with a 12-month DMF S increment greater than or equal to 2 or = 0 were also formed from the two groups, respectively. Measurements were made with an iridium oxide electro de inserted between teeth 13/14, 23/24, 34/35 and 44/45, before and every 5 min for 30 min after rinsing with 10% sucrose, and the 4 resulting 'Stepha n curves' averaged using a plaque pH analysis program. Supragingival plaque was collected from buccal and lingual smooth surfaces of posterior and upp er anterior teeth and its acid extract analysed for Ca, P and F. Caries-fre e subjects (based on past experience) had a significantly higher maximum pl aque pH and pH value after 30 min (reflecting a faster return to resting pH ), a lower minimum enamel dissolution capacity of plaque and recorded less time below pH 7.0 than did high-caries subjects. No other differences were significant, including those of the principal acidogenic parameters 'minimu m pH attained after a sugar rinse', 'curve area below the critical pH of 5. 5' and 'time below the critical pH'. Selection of the caries groups on the basis of both experience and incidence did not reveal significant differenc es in more parameters. Upper arch plaque was significantly more acidogenic than lower arch plaque, and there was a consistently strong association bet ween upper and lower arch values in individuals. Ca, P and F in the subject s' plaque had little or no influence on the principal acidogenic parameters . Our failure to find a relationship between caries prevalence or activity and these principal acidogenicity parameters may be related to differences between fissure and smooth surface plaque, temporal variations in acidogeni city and/or to use of F toothpaste during the 1-year observation period. Th ese results support the view that factors such as the frequency of acidogen ic episodes may be more important in caries progression than the degree of acidogenicity during any one episode.